Shallow Foundations

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					Shallow Foundations

       Ch. 9
                 Introduction
• Definition of a foundation
  – shallow (footings)
  – deep (pier, caisson or piles)
• Footings - Fig. 9-2
  –   individual
  –   continuous or wall
  –   strap
  –   mat foundation
            Foundation Design
• Resistance to bearing capacity failure
• Resistant to excessive settlements
  – Table 9-1
• Footing design procedure
  – p.264
         Loads on Foundations
•   dead load
•   live load
•   wind load
•   snow load
•   earth pressure
•   water pressure
           Depth and Location
• Frost action
  – effect of expansion and contraction
• Soil Volume change
  – depth of 5 to 10 ft. below surface
• Groundwater
  –   construction (dewatering)
  –   reduced strength
  –   hydrostatic uplift
  –   frost action
  –   waterproofing problems
      Bearing Capacity Analysis
•   Ultimate bearing capacity
•   allowable bearing capacity
•   factor of safety
•   Terzaghi equations
    – Continuous or wall footing(width B)
    – Circular footing(radius R)
    – Square footing(width B)
• Terzaghi bearing factors
  – Figure 9-7 bearing capacity factors for both
    cohesive and cohesionless soils
     • cohesion (C) and angle of internal friction ()
     • cohesive soils,  is equal to zero
     • cohesionless soil, c is equal to zero
  – dense sand and stiff clay
  – loose sand and soft clay
     • c’ and ’ values
• Figure 9-9 bearing capacity factors for
  cohesionless soil
  Effect of groundwater table on
         bearing capacity
• Case 1 - groundwater at or above base of
  footing
• Case 2 - groundwater is at a distance, B,
  below the base of the footing
• Case 3 - groundwater is between the base of
  the footing and a distance B below the base
  of the footing
  Structural Design of Footings
• Size of Footing
  – Section 9-5
  – Required base area of a footing is obtained by
    dividing the Column Load, Q, by the allowable
    bearing capacity, q(a).

				
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